Injection molded articles are used for a variety of purposes. Plastic injection molded products are commonly made from materials such as polyethylene (PET) or polypropylene (PP). These products resist environmental degradation, and are reasonably durable, watertight, and economically produced.
However, plastic materials such as PET and PP are gas (e.g., oxygen, nitrogen, etc.) permeable. For applications in which gas permeability is undesirable, for example, food products, medicines and products that degrade upon gaseous exposure, a barrier material or scavenger material is co-injected with the plastic material. Typically, the barrier material, such as Ethyl Vinyl Alcohol (EVOH), is injected as an interior core material stream between an inner and outer flow stream of the PET or PP material stream, forming an EVOH interior layer inside the PET or PP skin to form the molded product. In order to prevent detrimental gas permeation, it is necessary that the interior barrier layer extend throughout substantially the entire portion of the molded article that is exposed. Even if a very small percentage of the exposed surface area lacks an adequate barrier layer, detrimental amounts of gas permeation may occur.
In order for the barrier layer to form throughout the molded article, it is necessary that the interior layer material flow to substantially the ends of the mold cavity during the molding process. If the interior layer material does not flow to the end of the cavity during molding, there will be an inadequate barrier layer at the corresponding “end” of the molded product. On the other hand, if the interior layer material flows too quickly, the interior layer material can penetrate or breakthrough the flow front or leading edge of the inner and outer layer material (i.e., skin), causing undesirable results. Thus, known techniques attempt to cause the interior layer material to flow to the ends of the mold cavity without breakthrough, e.g., by attempting to precisely control injection parameters, such as, for example, injection pressure, temperature, timing, injection location, etc.
Due to significant material flow disparities in non-symmetric mold cavities (i.e., non-symmetric molded articles), this co-injection process has until now been limited to products that are essentially symmetrical in shape. However, even symmetrical geometries, which theoretically have symmetrical flow characteristics throughout the mold cavity, have met with limited success. Systemic and process variations (e.g., manufacturing tolerances in mold cavity dimensions and surface finishes, local temperature variations, injection pressure variations, normally occurring streamline variations, limitations of calculation methodologies used, etc.) cannot be eliminated using current technology and can result in either breakthrough or “gaps” (or both) in barrier layer coverage. Thus, previously known techniques do not provide adequate and/or consistent permeability resistance.
Accordingly, there is a need for methods and apparatuses for forming injection molded articles having an interior layer where the interior layer extends sufficiently throughout the article to adequately prevent gas permeation without breakthrough. There is further a need for injection molded articles containing such a layer.